This invention relates generally to integrated multi-frequency communication passive components made by micro-electromechanical (MEM) techniques, more particularly, it relates to a MEM process for fabrication of integrated multi-frequency communication passive components by fusing a xe2x80x9cflip-chipxe2x80x9d approach into low-temperature co-fired ceramics so as to produce a low-cost, high-performance, and high-reliability hybrid communication component.
Thanks to the speedy progress of micro-electromechanical (MEM) techniques, size of wireless communication systems have been significantly minimized in recent days, and xe2x80x9ccommunication system on a single chipxe2x80x9d is considered a trend ready on the road. For example, the xe2x80x9cWatch phonexe2x80x9d of Samsung Korea presented to the U.S. market for transmitting wireless voice or e-mail is a phone set in volume slightly larger than a conventional watch, and Hewlett Packard, a widely known company of communication equipments and medical instruments, works together with Swatch, a Swiss watch maker, for research and development of a new watch capable of monitoring wireless network and medical care. The examples already tell a new track for the communication single chip to sprint forward.
However, a single chip is so far incapable of handling a wireless communication system by itself alone when lacking some cooperative components, such as the surface acoustic wave (SAW) filter, quartz oscillator, microswitch, etc., and some on-chip inductors and variable capacitors disposed by using MEM techniques to minimize volume of the communication system and reduce assembly cost thereof.
In reviewing the chip-fabrication process, the CMOS fabrication process is considered workable to satisfy systems below 5xcx9c6 GHz (oscillation components already up to 9 GHz) while that of Bipolar or GaAs device is applicable to systems in higher frequencies, and meanwhile, MEM techniques are progressed upwardly to try breaking through the critical watershed. So far, Clark T. C. Nguyen and Linda P. B. Katehi in Electrical Engineering Department of the Michigan University may be recommended as the global typical representative figures for research of MEM communication components, wherein the former aims at the components below 2.5 GHz while the latter at 20 GHz up; and both use silicon substrate.
In a long-range consideration, the fabrication process of MEM wireless communication components below 10 GHz will probably go compatibly with the CMOS process, otherwise, the process may become more complicated and the original electrical parameters as well as the yield rate may be deteriorated to some extent that would suggest to adopt discrete components instead. While in a short-range viewpoint, the components above 20 GHz are still a domain of GaAs and compounds of groups III and V in the periodic table of elements.
This invention is positioned and oriented in developing MEM microwave component that takes low-loss substrates, such as Al2O3 ceramic substrates, glass substrates, etc. in order to lessen signal attenuation, but, as a matter of fact, the developed techniques of this invention are offered with generalities for being applied in multi-frequency bands.
A monolithic microwave integrated circuit (MMIC) is usually merited in: LTSS (light, thin, short, small) measurements, low-cost mass production, wide bandwidth, high reliability, and multi-purpose combination, however, unfortunately, it is defective in: after-adjustment impossible and a relatively longer development time.
In a U.S. Pat. No. 6,049,702, the mentioned techniques regarding an integrated passive transceiver section and MEM fabrication process thereof are found imperfect as described below:
1. MEM techniques are not integrated for application to low-temperature co-fired ceramics, therefore, some low-accuracy passive components for cost saving are not easy to obtain.
2. Only MEM surface micromachining techniques are taught in embodiments for fabrication of capacitors, inductors, and microswitches, application to multi-layer structure, such as antennas or low-frequency filters, and the packaging compatibility is doubtful.
3. Possible coupling interference when integrating and miniaturizing is obviously not yet put into consideration that would inevitably affect the integration density in lack of isolation design.
4. Both materials used and temperature conditions in the fabrication process are different from that of the CMOS process, therefore, such a process may not be a post process to that of the CMOS.
5. Different design methods for different frequency-bands haven""t been proposed that wouldn""t be capable of controlling circuitry area properly.
In view of above said imperfections, after years of constant efforts in research, the inventors have consequently developed a micro-electromechanical (MEM) process for fabrication of low-cost, high-performance, and high-reliability hybrid communication components.
The primary object of this invention is to provide a micro-electromechanical (MEM) process for fabrication of integrated multi-frequency communication passive components by fusing MEM techniques into low-temperature co-fired ceramics so as to produce low-cost, high-performance, and high-reliability hybrid communication components, to which high accuracy is a surplus.
Another object of this invention is to provide a micro-electromechanical (MEM) process for fabrication of integrated multi-frequency communication passive components, wherein MEM surface micromachining techniques are applied for disposing capacitors, inductors, microswitches, etc., on a substrate; meanwhile, MEM three-dimensional techniques and the hot-embossing or injection molding method are employed for forming an antenna, a shielding top cover, etc., on another substrate to realize a multi-layer structure.
Yet another object of this invention is to provide a micro-electromechanical (MEM) process for fabrication of integrated multi-frequency communication passive components, wherein coupling interference to be possibly incurred by integrating and miniaturizing is also put into consideration so that the isolation design and corresponding process are prepared.
Yet another object of this invention is to provide a micro-electromechanical (MEM) process for fabrication of integrated multi-frequency communication passive components, wherein materials applied and temperature conditions in the process are compatible with that of the CMOS fabrication process, hence, this MEM process may serve for a post process of the latter.
Yet another object of this invention is to provide a micro-electromechanical (MEM) process for fabrication of integrated multi-frequency communication passive components, which would propose optimum design methods in response to different frequency-band requirements, particularly for the antennas, filters, inductors, etc., to keep the circuitry area indifferent to change of wavelength.
In order to realize abovesaid objects, the MEM process of this invention has integrated passive components in an IC device, whose main architecture is built on a low-loss substrate such as an alumina substrate or a glass substrate, for example, while a low-cost plexiglass plate or a plastic plate made of Poly Methyl Methacrylate (PMMA), for example, may serve as an upper-layer substrate for disposition of an antenna and protection or passivation of a component.
Taking the glass-made substrate for instance, the processing procedure is to firstly perforate a substrate to form via holes, then prepare a simple etching mask for exposing and developing. An etching process on the glass substrate could be made by using Hydrofluoric (HF) acid or by reacting ion etching (RIE), in which etching material is optional for avoiding destruction of a front panel or creation of a undercut in form of an air cavity on the substrate. When such an air cavity is deeper than 30 xcexcm, it can be processed mechanically by using a tungsten-oxide cutting tool or sand blaster. The grounding layout and a feeding structure on back of an antenna are to be masked together with filters, switches, resistors, capacitors, etc., on the lower low-loss substrate before undergoing etching. Then a metallic layer (Cu) is electroplated on the selfsame low-loss substrate to accomplish a single plane-layer framework containing the antenna feeding lines, filters"" input lines, and filters"" output lines.
In the case of a PMMA substrate, an upper-layer substrate may be formed by hot-embossing molds.
The purpose of forming an air cavity on the substrate by etching and hot-embossing is to reduce the dielectric constant of the substrate and obtain a wider bandwidth, and this formality may be omitted should the dielectric constant of the substrate itself is relatively low.
Moreover, in order to match with different frequencies and measurements of the component, a double-layer structure will do with respect to low-frequency applications while a double- or single-layer structure is fit for mid- or high-frequency applications because of the relatively smaller area of the component, wherein the silver epoxy may be applied in-between layers; the isolation design of the component will confine xe2x80x9ccoupling effectxe2x80x9d under some specified states; and by taking advantage of a xe2x80x9cflip-chipxe2x80x9d approach and low-temperature co-fired ceramics, the component of this invention can be integrated to become a low-cost, high-performance, and high-reliability hybrid communication component.
Methods for fabricating related electronic parts are described below individually.
1. About xe2x80x9cvia holexe2x80x9d
xe2x80x9cVia holexe2x80x9d is a very important issue in microwave-circuitry, particularly when most ICs are using a microstrip-line structure. The primary object of the xe2x80x9cvia holexe2x80x9d is to tightly join a front-surface ground and a back-surface ground together, otherwise, the equivalent inductance caused by the interval between those two grounds will descend the circuit gain and meanwhile consume more power.
The xe2x80x9cvia holexe2x80x9d is made by perforating a through hole on the backside of the substrate and penetrating through the ground metal on the other (front) side, then the backside is plated to have a copper (Cu) layer attached. To a silicon substrate, an exposure machine for aligning the photo mask on both sides and a charge-coupled device (CCD) camera are required to display the image of backside on monitor that enables an operator to align and adjust position of the masks for exposing. The boring ways according to different substrate materials are listed in the table shown below:
A generic chemical plating or electroplating process may be adopted for coating or filling metal in the via hole.
2. About Resistor
In the microwave circuitry, a resistor is usually employed in a bias circuit, a voltage-dividing circuit, or a feedback circuit, in which the temperature coefficient of resistor is the major factor that can affect the circuit""s stability under a large-current operation. A metallic thin film resistor has a relatively lower temperature coefficient while those made of the active layer of semiconductor have temperature coefficients at values far exceedingly over that of the metals. The related parameters of thin film resistor in different materials are referenced in the table shown below:
3. About Solenoid Inductor
An associated air-bridge is usually needed in fabricating a plane inductance for jumping connection, meanwhile, the metallic wire used must be fat enough to reduce ohmic loss in transmission. Hence, in addition to producing a seeding layer by vapor deposition (or electronic beam deposition or sputtering), the metallic wire is further plated to form a thick copper layer and is formed in square for easy fabrication of the photo mask. As the Q value is fluctuated according shape of an inductor and for obtaining a high Q of this invention under various frequencies, the inductor structure of this invention is basically a solenoid inductor. When the inductor is applied under a low frequency, a magnetic body may be inserted in the solenoid coil, which may be substituted by a body with low dielectric constant in the case under a high frequency.
4. About Capacitor
A metal-insulator-metal (MIM) capacitor has some parameters in an insulation layer shown in the table below:
5. About Antenna
As indicated in FIG. 3, an antenna major structure constructed with MEM techniques is built on a low-loss substrate, wherein a low-cost plexiglass or a polymer substrate, in PMMA for example, may be chosen to form an upper layer, in which:
(I) In case the glass substrate is taken, the forming procedure of the antenna structure is to firstly perforate xe2x80x9cvia holesxe2x80x9d in a wafer; then prepare a simple etching mask for exposing and developing; perform etching process by using Hydrofluoric (HF) acid or by reacting ion etching (RIE) while leave the antenna backside grounding and the pattern of the antenna feeding structure to be masked together with filters, switches, resistors, capacitors, etc., on the lower ceramic substrate before undergoing etching; and electroplate a metallic layer (Cu) on the selfsame low-loss substrate to accomplish a single plane-layer framework containing meanwhile the antenna feeding lines, filters"" input lines, and filters"" output lines;
(II) In case the PMMA substrate is taken, the upper-layer substrate may be formed by hot embossing or injection molding; the purpose of forming an air cavity on the substrate by etching and hot embossing is to reduce the dielectric constant of the substrate and obtain a wider bandwidth while this forming step may be omitted should the dielectric constant of the substrate itself is relatively low; and a xe2x80x9cvia holexe2x80x9d is provided to the short-circuit end with an extra resistor if desired to thereby obtain an optimum bandwidth and a patch antenna.
6. About Filter
In the microwave frequency range, it""s nothing peculiar to have the circuitry layout of filter amended or changed in response to the size, function, process difficulties, and cost of the filter. When a filter of this invention is employed for low-frequency application (0.9 GHzxcx9c5 GHz), the odd/even mode impedance concept for circuitry analysis, a hybrid circuit comprising xe2x80x9cdistributedxe2x80x9d and xe2x80x9clumpedxe2x80x9d circuits, and the microstrip-line structure for transmission are all adopted as the coplanar waveguide (CPW) structure applicable to a high-frequency range (usually 30 GHz or up) doesn""t fit the mentioned range.
7. About Microswitch
The microswitch adopted in this invention is basically a cantilever structure, comprising: an upper electrode set above the cantilever; a contact-electrode set at the end of the cantilever; a lower electrode disposed on the substrate in the vertically projected shadow of the cantilever at a position corresponding to the upper electrode; and a signal-discontinuity line. When a static-electricity force is applied to the upper or the lower electrode alternatively, the cantilever is attracted to contact the lower electrode, for example, to close the circuit, or, on the other hand, the cantilever is restored to let the contact-electrode open the circuit as soon as the static-electricity force is removed.
The dielectric constant of a LTCC package material is normally equal to 4xcx9c8, which would allow a wider microwave transmission line to work that results in a lower conduction loss compared with a transmission circuit on a Si, GaAs, or Alumina substrate. Furthermore, the LTCC has a small amount of loss tangent about 0.002 at 10 GHz and has a relatively lower dielectric attenuation accordingly. The LTCC package is commonly composed of multiple ceramic layers of 0.1xcx9c0.15 mm thick each, and between layers, transmission lines and a part of passive components may be printed. The components on each layer may be interconnected by opening via holes with a laser beam or by mechanical perforation before being sintered so as to minimize the package and complete the RF transmission lines, bias lines, and control lines.
Whereas, as high or low accuracy is indifferent to some passive components implemented in a communication system, therefore, it seems reasonable and worthy of integrating the MEM approach to low-temperature co-fired ceramics by setting via holes in a low-loss substrate and forming some bumps on back of the substrate to have the bumps connected with the circuit and the soldering pad in an upmost layer on the substrate to form a xe2x80x9cflip-chipxe2x80x9d package for cost-saving.
In fabrication of integrating circuits, a high-density integration of transmission lines or circuit disposition is known liable to incur coupling or cross talk. For improving isolation of package to get rid of coupling or cross talk, following policies may be taken independently or combinedly:
1. Try reducing radiation source and discontinuity in connection between transmission lines and components.
2. Try using metallic shield for besetting wave radiation.
3. Try using material of high dielectric constant for channeling wave radiation.
Regarding the policy of using metallic shield for besetting wave radiation, please refer to the literaturexe2x80x94G. E. Ponchak, et. al. xe2x80x9cThe use of metal filled via holes for improving isolation in LTCC RF and wireless multichip packages,xe2x80x9d IEEE Tran. Advanced Packaging, Vol. 23, No. 1, February 2000, pp. 88-99-in which LTCC is mentioned for being made in dual-line via holes connected with strip lines to form fences to confine electromagnetic radiation. This policy would entail a great expenditure with limited efficacy.
Besides, in another literaturexe2x80x94R. F. Drayton, et. al., xe2x80x9cMonolithic Packaging concepts for high isolation in circuits and antennas,xe2x80x9d IEEE Tran. Microwave Theory and Techniques, Vol. 46, No. 7, July 1998, pp. 900-906xe2x80x94in which an upper cover is mentioned for being etched to form cavities corresponding to microstrip lines, and the inner wall of every cavity is plated with metal and grounded, or a part of the substrate right under the microstrip lines is etched to thereby enclose electromagnetic radiation of the microstrip lines. This is another besetting technology too.
A xe2x80x9cchannelingxe2x80x9d policy suggests to confine most part of electromagnetic radiation in a high dielectric constant material with a negligible amount disappeared in the air.
In short, this invention is designed to waive connection discontinuity between the transmission lines and components, and perform the policy of xe2x80x9cbesettingxe2x80x9d and xe2x80x9cchannelingxe2x80x9d combinedly to eliminate or reduce effect of coupling and/or cross talk.